इति श्रुतेः ।ऐश्वर्यं प्रतापो वा तपो महदस्येति वा महातपाः ॥
In plain words
That is the śruti speaking. And tapas here means sovereign power — or blazing majesty. Or read it simply: his tapas is great. Hence the name, Mahātapāḥ.
Close to the Sanskrit
Thus, from the śruti. Tapas is sovereignty, or else burning majesty; or, 'his tapas is great' — thus Mahātapāḥ.